1. Field of the Invention
The invention relates to a hearing aid with a signal processing unit and at least two microphones which can be coupled together to form directional microphone systems of a different order. The invention further relates to a method for operating a hearing aid.
2. Description of the Related Art
Hearing aids with at least two microphones for obtaining directional microphone characteristics of a first or higher order are known in the prior art. When directional microphone systems of a second or higher order are used, an unwanted drop in the directivity index (DI) occurs in individual frequency ranges of the input signal.
In hearing aids, the frequency range of 100 Hz to 6 kHz is of particular interest in improving hearing. Using this frequency range in directional microphone systems of a first order, a directivity index is obtained which falls slightly in the direction of higher frequencies. At lower frequencies, for example up to 1 kHz, DI values of about 5 dB are obtained. However, because of the high degree of sensitivity to component tolerances, directional microphone systems of n-th order with n>1 have a negative directivity index at low frequencies. However, DI values of 7 dB and more can be achieved for frequencies of 1 kHz to 5 kHz. In order to be able to obtain higher DI values for low frequencies too, it is necessary to keep to narrow component tolerances (e.g., the phase difference of the microphones to <0.25°) which can best be achieved with silicon microphone arrays. However, at the supply voltage (<1V) used for hearing aids, these arrays have too great of a signal-to-noise ratio, which makes them not yet practicable.
U.S. Pat. No. 5,757,933 discloses a hearing aid in which it is possible to switch manually between a microphone of zero order (a microphone without directivity) and a microphone system of first order. In this device, the person wearing the hearing aid performs the switching.